The present invention relates to a decision feedback equalizer (DFE) for removing intersymbol interference (ISI) which is particular to pulse transmission.
A DFE is one of the implementations heretofore proposed for the removal of ISI which occurs during pulse transmissions. In a DFE, an adaptive filter having taps corresponding to a length over which ISI has as influence is installed to generate estimated ISI, so that the actual ISI occurring during the transmission of pulses over a channel may be suppressed. The tap coefficients of the filter are sequentially corrected or updated by determining the correlation between the predicted residual ISI of a received signal.
A problem with a DFE is that the adaptive operation is unachievable unless the residual ISI obtain residual signal which is produced by subtracting estimated ISI from a received signal having ISI, is accurately detected to enable the correction of the coefficients. For example, where a biphase or a two-level code which will be described later is used as a transmission line code, extracting just the ISI is impractical because the received signal level has no zero level duration due to the particular nature of two-level codes; the DFE thus fails to correct the tap coefficients.
A solution to the above problem is proposed in a paper entitled "Some Considerations on the Design of Adaptive Digital Filters Equipped with the Sign Algorithm", IEEE Transactions on Communications, Vol. COM-32, No. 3, March 1984, pp. 258-266. The approach as described in this paper is to equip a DFE with a subtractor and an automatic gain control (AGC) circuit to remove all signals except the ISI. Such a scheme, however, undesirably scales up the hardware requirements partly because the AGC circuit is essential and partly because a complicated control is necessary for a signal which is fed from the AGC to the subtractor and free from ISI to be maintained at an adequate level.